Electron guns



Sept. 6, 1966 A. REDDISH ETAL ELECTRON GUNS Filed Jan. 5, 1963 .E Illll FigZ 2./ m al@ EMA@ l/l//l-L/n/v/ ffl/@vw BYGL/ L C? HTTORNEYS United States Patent O 3,271,609 ELECTRON GUNS Alan Reddish, Pinner, and Donald William Ward, Watford, England, assignors to rElie lvl- Valve Company Limited, London, England Filed Jan. 3, 1963, Ser. No. 249,259 Claims priority, application Great Britain, Jan. 116, 1962, 1,632/62 2 Claims. (Cl. 313-82) This invention relates to electron guns, that is to say arrangements of electrodes for generating electron beams in electron tubes.

The invention is concerned in particular with crossed field electron guns, by which is meant electron guns of the kind including a thermionic cathode and a further electrode (hereinafter referred to as the plate) which between them `define a gun space which is laterally bounded by the electron emissive surface of the cathode and the operating surface of the plate, the gun being arranged to operate with the plate held positive with respect to the cathode so as to establish an electrostatic iield within the gun space and with a magnetic tield established within the gun space perpendicular to the electrostatic lield and to the length ofthe gun space, the geometry of the `electrodes being such that the gun space is of uniform cross-section in planes perpendicular to -such magnetic field, whereby in operation electrons emitted from said emissive surface are formed into a beam which travels along the gun space and then emerges from one end of the gun space. Electron guns of this kind are commonly used in M-type travelling wave tubes.

ln certain cases it may be desired to make the emissive surface of the cathode of a crossed field electron gun relatively long, for example in order to obtain a particular total beam current in an electron tube in which the lateral dimension ofthe emissive surface is limited by considerations relating to the operating frequency range of the tube. If, however, one uses the conventional arrangement in which the spacing between the emissive surface of the cathode and the operating surface of the plate is uniform along the length of the gun space, we have found that difficulties may arise in the operation of an electron tube in which the electron gun is installed if the length of the emissive surface of the cathode is made greater than the spacing between the emissive surface of the cathode and the operating surface of the plate. These diiculties, which may result in erratic performance of the tube or the generation of excessive noise in the tube, appear to be associated with the occurrence of a non-uniform distribution of electron emission density along the length of the emissive surface when the electron gun is operated so that the electron emission is space charged limited, as is otherwise desirable in order to avoid the performance of the tube being unduly dependent on the heating of the cathode.

It is therefore an object of the present invention to provide a crossed ield electron gun having a geometry such than the difliculties referred to above may be alleviated.

The invention will be further described with reference to the accompanying drawings, in which:

FIGURE 1 is an explanatory diagram relating to the geometry of crossed iield electron guns; and

FIGURE 2 is a diagrammatic sectional view of part of the electrode system of an Mtype travelling wave fube adapted to operate as a backward wave oscillator.

Before defining an electron gun in accordance with the invention, it is desirable to clarify certain concepts relating to the geomery of crossed eld electron guns, particularly having regard to cases in which the spacing between the emissive surface of the cathode and the operating surface of the plate is not uniform along the length of the gun space. This geometry is essentially two-dimensional, since the gun space is of uniform cross-section perpendicular to the appropriate magnetic field, and the relevant parameters may therefore be defined in terms of lthis cross-section, as illustrated in FIGURE 1 of the drawings.

The traces in this cross-section of the emissive surface of the cathode and the operating surface of the plate are respectively denoted C and P. We define the longitudinal axis (denoted A) of the cross-section as that straight line which is tangential to C at the forward end E of C (that is the end corresponding to the end of the gun space from which the electron beam emerges in operation); it will be appreciated that A will coincide with C if the latter is a straight line, for example where the emissive surface is planar or cylindrical. The position of any point O lying on the projection of C on A will be defined by a parameter X, equal to x/L, where x is the length of the line OE and L is the length of the projection of C on A. For a given value of X, the spa-cing between the emissive surface of the cathode and the operating surface of the plate is then defined in terms of a parameter Y, equal to y/D, where y is the distance between C and P along a straight line which is perpendicular to A and which passes through the point O corresponding to the given value of X, and D is the distance between C and P along a straight line which is perpendicular to A and which passes through E. lt will be appreciated that for a conventional crossed eld electron gun, the value of Y is one -for all values of X from zero to one. Finally, F is deiined as the distance (which may be zero) by which P extends beyond E in a direction parallel to A, and S is the spacing between the trace C and the trace P at any point B on the trace C measured along the normal to the trace C at that point B.

According to the present invention, a crossed field electron gun has a geometry such that, in terms of the parameters L, D, X, Y, F and S deiined above: L is greater than D; for values of X between zero and 0.7 inclusive the value of Y lies between one and (1}l.5X) inclusive; for values of X between 0.7 and one inclusive the value of Y is not less than (2.5X-0-75), the value of Y being iinite for values of X up to at least 0.9; for any value of X `for which the value of the diiiferential coethcient dY/dX is finite there is no greater value of X for which the value of rz'Y/dX is lower; F is considerably less than D; and S never decreases with increasing values of X.

With such a geometry the uniformity of the electron emission ldensity along the length of the emissive surface under space charge limited conditions is improve-d as compared with the case of a conventional crossed field electron gun. From this point of view, it can be shown that ideally the geometry should be such that Y varies with X according to the equation Y=(1 X)*`/2; this would, however, result in a somewhat complicated shape for at least one of the electrodes of the electron gun, and in practice satisfactory results can be obtained with a simpler form of variation such that both C and P can be made rectilinear. In particular, satisfactory results can be obtained if the geometry is such that Y varies with X approximately in accordance with the equation Y=(lH-X).

One arrangement in accordance with the invention will now be described by way of example with reference to an M-type travelling wave tube adapted to operate as a backward wave oscillator, part of the tube being shown in a sectional View in FIGURE 2 of the accompanying drawings.

The tubeis generally of conventional form, having an evacuated met-al envelope (not shown) within which are mounted a delay line l, a sole 2, and an electron gun constituted by an indirectly heated thermionic cathode 3 and a plate 4. The delay line 1 is of the conventional interdigital type and comprises a pair of copper combs respectively having teeth 5 and 6 which are intercalated with each other, all the teeth 5 and 6 being identical except for the end tooth 7 which is thicker than the others. The two combs are secured to one face of a copper block 8 which is formed integral with part of the envelope, so that the teeth 5 and 6 extend across the mouth of a groove 9 of rectangular cross-section formed in the block 8, the spines of the two combs extending immediately alongside the groove 9 respectively on opposite sides of the groove 9 and each tooth 5 or 6 extending perpendicularly from the relevant spine nearly to the farther side of the groove 9.

The delay line 1 is provided at one end with an output connector in the form of a hollow metal waveguide 10 of rectangular cross-section, one end of the waveguide 10 being disposed in an aperture in the block 8 so that the interior of the waveguide 10 communicates with the space within the groove 9 and the other end (not shown) being sealed through the envelope of the tube. Three of the walls of the waveguide 10 terminate flush with the base of the groove 9, but the fourth wall is extended to form a slanting projection 11 which closes the groove 9 and whose end surface lies in the same plane as those faces of the teeth 5 and 6 which are farthest from the block 8. The waveguide 10 is provided with a longitudinally extending ridge 12 which projects from one of `the broader walls of the waveguide 10, the ridge 12 being extended to form a stub 13 whose free end is secured to the end tooth 7. At its other end the delay line 1 is provided with a retiectionless termination (not shown), for example by coating some of the teeth 5 and 6 at this other end of the delay line 1 with a resistive material.

The sole 2 is in the form of a molybdenum strip which is supported from the block 8 by means of an insulating support member 14, the sole 2 being disposed opposite the delay line 1 with one of its main faces parallel to, and spaced 0.6 millimetre from, those faces of the teeth 5 and 6 remote from the block 8. Thus between the delay line 1 and the sole 2 there is formed an interaction space 15 through which an electron beam flows in operation of the tube, the interaction space 15 having a width (in a direction perpendicular to the plane of the section of FIGURE 2) of about 3 millimetrcs.

The electron gun is disposed opposite that end of the interaction space 15 corresponding to .the end of the delay line 1 to which the waveguide 10 is connected. The emissive portion of the cathode 3 is in the form of a tubular block 16 of porous sintered tungsten impregnated with alkaline earth metal compounds, the block 16 being carried by a forked metal bracket 17 whose arms engage the ends of the block 16 and having disposed within it a spiral resistance heater 18 one end of which is connected to the bracket 17 and the other end of which is connected to a further metal bracket 19. The block 16 has an external cross-section 5 millimetres square, and is disposed with its longitudinal axis parallel to the width of the interaction space 15 and with one of its lateral faces coplanar with and facing in the same direction as that face of the sole 2 which bounds the interaction space 15, the spacing between the coplanar faces of the block 16 and the sole 2 being 0.75 millimetre. The block 16 has wrapped round it a metal foil 20 which covers the lateral surfaces of the block 16 except for a rectangular area 3 millimetres wide which extends all the way across that face of the block 16 which is coplanar with a face of the sole 2, the foil 20 serving to inhibit electron emission from the block 16 except over the exposed area, which constitutes the emissive surface 21 of the cathode 3. The width of the emissive surface 21 is aligned with that of the interaction space 15, and the edges of the foil 20 adjacent the ends of the surface 21 arer made liush with this surface.

The plate 4 is in the form of a tantalum strip 2.5 millimetres wide, which is disposed facing the surface 21 with its width parallel to, and symmetrically disposed with respect to, the width of the surface 21. The plate 4 extends opposite the whole length of the surface 21, and at one end extends 0.25 millimetre (measured in a direction parallel to the longer sides of the surface 21) beyond the forward end of the surface 21 (that is the end nearer the sole 2); this slight extension of the plate 4 beyond the end Of the surface 21 is desirable in order to shield the surface 21 from the member 11, which is maintained in operation at a high positive potential with respect to the cathode 3. The plate 4 is inclined with respect to the surface 21 so that the spacing between the surface 21 and the nearer main face 22 of the plate 4 (which constitutes the operating surface of the plate 4) varies linearly along the length of the surface 21 from a value of 0.45 millimetre at the forward end of this surface to a value of 0.9 millimetre at the other end of this surface. It will be appreciated that in this embodiment of the invention the gun space 23 between the surfaces `21 and 22 is of uniform cross-section in planes perpendicular to the shorter sides of the surface 21, and that in terms of the parameters described above, L=11.1D, C coincides with A, C and P are straight lines such that Y=(li-X) for all values of X from zero to one, and F is equal to 0.56D.

In order to inhibit any sideways escape of electrons from the gun space 23 in operation, there are provided a pair of further metal strips 24, parts of which are respectively disposed on either side of and coplanar with the plate 4 so as to be slightly spaced from the plate 4, and parts of which are bent over and secured to the bracket 17 so that in operation the strips 24 are maintained at the potential of the cathode 3.

The plate 4 and the brackets 17 and 19 are supported by an insulating support member 25 which is in turn supported by a bracket 26 secured to the block 8. Separate leads (not shown) are connected to the sole 2, the plate 4 and the brackets 17 and 19, these leads being sealed through the envelope in an insulated manner.

In operation the tube is disposed between the pole pieces of a permanent magnet (not shown) so that the interaction space 15 and the gun space 23 lie in a uniform magnetic field H of strength 5,000 oersted directed parallel to the shorter sides of the surface 21, the sense of the field H being such that an electron in the gun space 23 travelling -away from the surface 21 in a direction perpendicular to the surface 21 will experience a force due to the field H directed parallel to the longer sides of the surface 21 and towards the forward end of the gun space 23. Taking the potential of the cathode 3 to be zero, the other electrodes are operated at potentials as follows: the sole 2, 2,000 volts negative; the delay line 1, 2,000-4,000 volts positive; the plate 4, up to 1,500 volts positive. A voltage is applied to the heater 18 such that the cathode 3 is heated to a suiiiciently high temperature to ensure that the electron emission is space charge limited.

The electrons emitted from the cathode 3 are formed, under the influence of the electrostatic and magnetic fields established in the gun space 23, into a beam which iows .along the gun space 23 towards its forward end and thence into the interaction space 15. The beam iiows along the length of the interaction space 15 under the influence of the crossed electrostatic and magnetic fields established in this space, and oscillations are generated in known manner by interaction between electrons in the beam and the field of an electromagnetic wave travelling along the delay line 1 in a sense opposite to the direction of the iiow of the electron beam. The amplitude of the wave is built up as it travels along the delay line 1, and an output is derived from the waveguide 10. The frequency of the oscillations may be varied by varying the potential applied to the delay line 1 over the range indicated above, and the beam current, and hence the output power, may be varied by varying the potential applied to the plate 4, the beam current corresponding to the maximum plate potential of 1,500 volts being about 300 milliamp'eres.

The performance of the tube described above is found to be considerably more reliable 'than is the case for a similar tube in which the plate 4 is replaced by a plate whose operating surface is disposed parallel to, and spaced 0.45 millimetre from, the emissive surface of the cathode.

We claim:

1. A crossed field electron gun including a thermionic cathode having an electron emissive surface and a plate having an operating surface which faces said emissive surface so that the cathode and plate `deiine between them a gun space which is laterally bounded by said surfaces, the gun being arranged to operate with the plate held positive with respect to the cathode so as to establish an electrostatic lield within the gun space and with a magnetic eld established within the gun space perpendicular to the electrostatic eld and to the length of the gun space, the geometry of the electrodes being such that the gun space is of uniform cross-section in planes perpendicular to such magnetic iield, whereby in operation electrons emitted yfrom said emissive surface are formed into a beam which travels along the gun space and then emerges from one end of the gun space, and said uniform cross-section being such that the parameters L, D, X, Y, F and S dened below satisfy the following conditions: L is greater than D; for values of X bet-'Ween zero and 0.7 inclusive the value of Y lies between one and (l-l-LSX) inclusive; for values of X between 0.7 and one inclusive the value of Y is not less than (2.5X-0.75), the value of Y being iinite for values of X up to at least 0.9; for any value of X for which the value of the differential coei'licient dY/dX is nite there is no greater value of X for which the value of dY/a'X is lower; F is considerably less than D; and S never decreases with increasing values of X, where, in said crosssection: L is the length of the projection of the trace C of said emissive surface on -an axis A constituted by that straight line which is tangential to the trace C at that end point E of the trace C which corresponds to the end of the gun space from which the electron beam emerges in operation; D is the distance between the trace C and the trace P of said operating :surface along a straight line which is perpendicular to the axis A and which passes through the point E; for any point O on said projection of the trace C, X is the distance between the points O and E divided by the length L, and Y is the distance between the traces C and P along a straight line which is perpendicular to the axis A and which passes through the point O divided by the distance D; F is the distance by which the trace P extends beyond the point E in a direction parallel to the axis A; and S is the spacing between the trace C and the trace P at any point B on the trace C measured along the normal to the trace C at that point B.

2. An electron gun according to claim 1, in which said cross-section is such that Y varies with X approximately in accordance with the equation Y=(1i-X) for all values of X from zero to one, X and Y being as dened in claim 1.

References Cited by the Examiner UNITED STATES PATENTS 2,807,739 9/1957 Berterottiere et al. 315-35 2,976,454 3/1961 Birdsall et al. 315-35 3,071,709 l/l963 Paananen et al. 3l5-3.5 3,084,278 4/1963 White 315-35 JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. A CROSSED FIELD ELECTRON GUN INCLUDING A THERMIONIC CATHODE HAVING AN ELECTRON EMISSIVE SURFACE AND A PLATE HAVING AN OPERATING SURFACE WHICH FACES SAID EMISSIVE SURFACE SO THAT THE CATHODE AND PLATE DEFINE BETWEEN THEM A GUN SPACE WHICH IS LATERALLY BOUNDED BY SAID SURFACES, THE GUN BEING ARRANGED TO OPERATE WITH THE PLATE HELD POSITIVE WITH RESPECT TO THE CATHODE SO AS TO ESTABLISH AN ELECTROSTATIC FIELD WITHIN THE GUN SPACE AND WITH A MAGNETIC FIELD ESTABLISHED WITHIN THE GUN SPACE PERPENDICULAR TO THE ELECTROSTATIC FIELD AND TO THE LENGTH OF THE GUN SPACE, THE GEOMETRY OF THE ELECTRODES BEING SUCH THAT THE GUN SPACE IS OF UNIFORM CROSS-SECTION IN PLANES PERPENDICULAR TO SUCH MAGNETIC FIELD, WHEREBY IN OPERATION ELECTRONS EMITTED FROM SAID EMISSIVE SURFACE ARE FORMED INTO A BEAM WHICH TRAVELS ALONG THE GUN SPACE AND THEN EMERGES FROM ONE END OF THE GUN SPACE, AND SAID UNIFORM CROSS-SECTION BEING SUCH THAT THE PARAMETERS L, D, X, Y, F AND S DEFINED BELOW SATISFY THE FOLLOWING CONDITIONS: L IS GREATER THAN D; FOR VALUES OF X BETWEEN ZERO AND 0.7 INCLUSIVE THE VALUE OF Y LIES BETWEEN ONE AND (1+1.5X) INCLUSIVE; FOR VALUES OF X BETWEEN 0.7 AND ONE INCLUSIVE THE VALUE OF Y IS NOT LESS THAN (2.5X-0.75), THE VALUE OF Y BEING FINITE FOR VALUES OF X UP TO AT LEAST 0.9; FOR ANY VALUE OF X FOR WHICH THE VALUE OF THE DIFFERENTIAL COEFFICIENT DY/DX IS FINITE THERE IS NO GREATER VALUE OF X FOR WHICH THE VALUE OF DY/DX IS LOWER; F IS CONSIDERABLY LESS THAN D; AND S NEVER DECREASES WITH INCREASING VALUES OF X, WHERE, IN SAID CROSSSECTION: L IS THE LENGTH OF THE PROJECTION OF THE TRACE C OF SAID EMISSIVE SURFACE ON AN AXIS A CONSTITUTED BY THAT STRAIGHT LINE WHICH IS TANGENTIAL TO THE TRACE C AT THAT END POINT E OF THE TRACE C WHICH CORRESPONDS TO THE END OF THE GUN SPACE FROM WHICH THE ELECTRON BEAM EMERGES IN OPERATION; D IS THE DISTANCE BETWEEN THE TRACE C AND THE TRACE P OF SAID OPERATING SURFACE ALONG A STRAIGHT LINE WHICH IS PERPENDICULAR TO THE AXIS A AND WHICH PASSES THROUGH THE POINT E; FOR ANY POINT O ON SAID PROJECTION OF THE TRACE C, X IS THE DISTANCE BETWEEN THE POINTS O AND E DIVIDED BY THE LENGTH L, AND Y IS THE DISTANCE BETWEEN THE TRACES C AND P ALONG A STRAIGHT LINE WHICH IS PERPENDICULAR TO THE AXIS A AND WHICH PASSES THROUGH THE POINT O DIVIDED BY THE DISTANCE D; F IS THE DISTANCE BY WHICH THE TRACE P EXTENDS BEYOND THE POINT E IN A DIRECTION PARALLEL TO THE AXIS A; AND S IS THE SPACING BETWEEN THE TRACE C AND THE TRACE P AT ANY POINT B ON THE TRACE C MEASURED ALONG THE NORMAL TO THE TRACE C AT THAT POINT B. 